Tweet
can some one explain how the full time VA hook up works? will i still set timing at idle with line plugged? what will timing at idle read after hooked up? i find my new L72 likes 4-6 BTC not 8. can i use the port in the manifold in front of the dist on the intake? thanks,bill
-
-
Re: BB vacum advance question
Always disconnect and seal/plug the vacuum source line to the vacuum advance unit when setting ignition timing. The port you mentioned can be used, it is usually used as a vacuum source for power brakes, auto-matic equipped Corvettes, and vacuum source for vacuum tank etc. Your carb should have a full time vacuum port, don't use a port that supplies vacuum only when the throttle is increased. You want a full time vacuum source. -
Re: BB vacum advance question
i asked because it appears the L72 carb does not have constant vacuum and an alternative source was reccommended many times in past post,billComment
-
Re: BB vacum advance question
bill------
The only "full time vacuum" source you'll find on your carb will be the source to which the choke unloader dashpot is connected. You can "T" into this source. You can also use the manifold vacuum fitting that you mention, but that means you'll have to completely re-route the distributor vacuum line.In Appreciation of John HinckleyComment
-
Re: joe BB vacum advance question
i just went from the dist to the port, will try teeing the other line. what i am curious of is timing. with adv disconnected and plugged timing reads 8 BTC at idle as spect. when i read it with line hooked up to constant vac it is way advanced at idle, about 850. it really likes this set up. idles nice, starts great, sounds strong. it was a good break in no grit in oil filter and valves were right on. does this timing set up sound like what i am looking for? bill p.s. got a foot of snow last night so no test drive for now she is my summer car but is cherry underneath and will keep like that.Comment
-
Re: joe BB vacum advance question
bill----
Yes, the way you've got it "is the way you're looking for".In Appreciation of John HinckleyComment
-
Re: thanks Joe i can finally sleep good tonight.
i do not know how guys like you can have more than one car going at the same time my one is all i can take. i also have 3 children and lots of guilt to deal with. glad guys like you are out there, billComment
-
Re: thanks Joe i can finally sleep good tonight.
With full time vacuum advance the TOTAL IDLE TIMING should be the sum of initial and full vacuum advance.
On some engines with quick centrifugal curves that start below idle speed, a few degrees of centrifugal is added to total idle timing.
According to the specs I have for L-72 the vacuum advance delivers a total of 15 degrees advance at 12" manifold vacuum. The centrifugal advance starts at about 900 and has 30@5000 max. So with 8 initial the total idle timing at no more than 900 should be about 23 degrees.
Since you never know what's been done to a 40 year old engine, it would be beneficial if you post the numbers stamped on your vacuum advance control bracket. You may have to remove the dist. cap to see it fully.
Also, it is a VERY GOOD idea to check the vacuum and centrifugal curves using a vacuum pump and dial back timing light to ensure that they are reasonably within spec.
In particular, the VAC must provide full advance at no less than an inch or two less than typical idle vacuum. If not idle quality and stability will likely be poor. So you need to know what you engine's typical idle vacuum is by measuring it.
The OE spec spark advance map (determined by VAC and centrifugal curve) for L-72 is okay, including those converted to full time vacuum advance, however, low and midrange torque can be enhanced by quickening the centrifugal curve with lighter springs up to the detonation limit, which may be possible depending on actual CR and fuel octane level.
DukeComment
-
Re: lost on 30@5000
i assume you mean 30 degrees advance and 5000 rpm and 900 is idle rpm, is that right? the engine is all rebuit to L72 specs comp ratio is around 11 net with work done. cam is L72 858 heads all redone. rebuilt K66 ign. to L72 specs. L72 carb. i calculate i should read around 23 with the constant vacuum my guess is that is what i read with the light. it idles at 850 nicley. thanks,billComment
-
Re: lost on 30@5000
The answer to your question is yes.
You will find the centrifugal and vacuum advance specs in you CSM or AMA specs. The centrifugal starts at about 900. It has nothing to do with idle speed. If you set your idle speed at 850 the centrifugal advance system should not provide any advance. If you set it at 1000, total idle timing will likely increase a few degrees because the centrifugal system is beginning to add advance.
What data is stamped on the vacuum advance control mounting bracket?
DukeComment
-
Re: lost on 30@5000
i will have to look. how does changing to constant vacuum change the scheme of things? if i read as set up by factory i get 8 btc at idle with line plugged and with constant vacuum at idle it reads what i would guess around 20. so i can get an idea of timing what would you guess my light should read with constant vacuum at idle?Comment
-
Re: lost on 30@5000
If your engine idles at the typical 850-900 @ 14" Hg. manifold vacuum for a L-72 and you have converted to full time vacuum advance and the vacuum advance control meets OE spec and the initial timing is set at 8 degrees, the total idle timing at no more than than 900 RPM idle speed should be about 8 +15 = 23.
If vacuum advance is ported the total idle timing will be only 8 degrees. This is less than the ideal timing at idle because at idle flame propagation is slow due to exhaust gas dilution, and because the idle timing is effectively retarded from ideal for this operating condition, EGT increases, which transfers additional heat into the cooling system as the exhaust gas traverses the ports, and the engine will run hotter or require greater heat dissipation through the radiator/fan system to maintain acceptable coolant temperature.
DukeComment
Comment